1. Field of the Invention
The invention relates generally to plasma-enhanced processing of a workpiece, e.g., a semiconductor wafer, and, more specifically, to an apparatus for improving the biasing and retention of a workpiece upon a workpiece support in a semiconductor wafer processing system.
2. Description of the Background Art
Plasma-enhanced processes have become increasingly important to the semiconductor industry, providing for precisely controlled thin-film depositions. For example, a plasma reactor in a high-temperature physical vapor deposition (PVD) semiconductor wafer processing system generally comprises a reaction chamber for containing a reactant gas, a pair of spaced-apart electrodes (cathode and anode) that are driven by a high power DC voltage to generate an electric field within the chamber, and a workpiece support for supporting a workpiece within the chamber. The cathode is a conductive plate coated with a target material that is to be sputtered or deposited onto the workpiece, while the anode is typically a grounded chamber component. The electric field, creating a reaction zone, captures electrons thereby ionizing the reactant gas into a plasma. The plasma, characterized by a visible glow, is a mixture of positive and negative reactant gas ions and electrons. Ions from the plasma bombard the negatively biased target releasing electrically neutral deposition material. As such, a conductive deposition film forms on the workpiece which is supported and retained upon the surface of the workpiece support. In such plasma reactors, the workpiece support is fabricated from a dielectric material such as ceramic. However, at wafer processing temperatures (typically in the range of 200-600.degree. C.) the resistivity of the ceramic is reduced and the workpiece support becomes more conductive. This characteristic is exploited to electrostatically clamp a workpiece, e.g., a semiconductor wafer, being processed to the surface of the workpiece support via the Johnson-Rahbek effect. A detailed explanation of the Johnson-Rahbek effect and an electrostatic chuck employing this feature is described in U.S. Pat. No. 5,463,526 issued Oct. 31, 1995 to Mundt herein incorporated by reference. This patent discloses a robust semiconducting layer covered by a thin insulating layer. When the semiconducting layer is biased, charges build up beneath the insulating layer. Since neither the semiconducting nor insulating layer is perfectly smooth, portions of the semiconducting layer are not totally covered by the insulating layer. As such, some of the charges pass from the semiconducting layer, through gaps in the insulating layer to workpiece retained by this electrostatic chuck. This charge migration phenomenon greatly increases the attractive force exerted on the workpiece.
To further enhance deposition in an ion metallization system, a specific type of PVD system, the substrate and pedestal are biased negatively with respect to the plasma. This is accomplished by providing RF power to the pedestal. A negative DC offset accumulates on the pedestal as a result of the higher velocity of electrons as compared to the positive ions in the plasma. In some processes, as neutral target material is sputtered from the target and enters the plasma, the target material becomes positively ionized. With the negative DC offset at the pedestal, the positively ionized target material is attracted to and deposits on the substrate more readily than on an unbiased pedestal. Ordinarily, a 400 KHz AC source is used to bias the pedestal, but other frequency sources such as a 13.56 MHz source may also be used.
Unfortunately, ceramic workpiece supports (also known as electrostatic chucks) cannot be used in RF enhanced PVD reaction chambers. The conductive nature of the ceramic at wafer processing conditions is beneficial to electrostatic chucking, but not to establishment of an effective RF induced electric field in the reaction zone. As such, other types of electrostatic chucks are used in RF enhanced chambers. For example, an electrostatic chuck consisting of a conductive layer, e.g., copper, encased in upper and lower insulating layers, e.g., polyimide, is adhered to a support surface of a pedestal. This type of electrostatic chuck is referred to as a Coulombic chuck since the mechanism of attraction of the workpiece to the support surface is a Coulombic force. Coulombic forces are usually weaker and do not retain a workpiece as well as electrostatic chucks employing the Johnson-Rahbek effect.
Consequently, there is a need for an apparatus that can electrostatically retain a workpiece with a ceramic chuck to take full advantage of the Johnson-Rahbek effect. Additionally, the apparatus must effectively bias the workpiece with RF power without compromising the integrity of the related RF electric field or the plasma in the processing chamber.
Therefore, there is a need in the art for an apparatus that optimally conducts RF power through a ceramic chuck.